The present invention relates to a drill bit, in particular for the drilling of rock. The invention further extends to roof bolts of the self-drilling kind.
Roof bolts are commonly employed in the underground mining industry to support the walls and/or the roof of excavated tunnels and openings against fragmentation and collapse. Roof bolts typically are applied by first drilling a hole in the rock wall and thereafter inserting the roof bolt therein. The bolt is fixed within the hole normally by a settable glue, such as a resin or a mortar cement.
Drill rods typically have a separate drill bit assembly, comprising a drill bit and a cemented carbide tip, which is fixed to the rod at one end thereof. The other end of the rod includes an arrangement for cooperating with a driving assembly, such as a hydraulic or pneumatic drill. The drill bit typically is cast from steel, and machined as required, and a cemented carbide drill tip is fixed to the drill bit, preferably concentrically, and extends normally beyond both the leading end of the drill bit and beyond the side periphery of the bit, to engage the rock and perform the cutting action during a drilling operation.
Typically a drill tip is bonded to one end of the drill bit, by welding, soldering or brazing, with brazing being most common. In the manufacture of a drill bit assembly, the drill tip is located in correct placement relative to the drill bit, whereupon the brazing operation, or other operation as required to bond the tip to the drill bit, is thereafter performed. Unfortunately however, the bonding operation often interferes with the correct placement of the tip relative to the drill bit and the resultant drill bit is formed with the drill tip non-concentrically located relative to the drill bit. While slight non-concentricity error is acceptable, more substantial non-concentricity error generally results in the drill bit being rejected.
Rejection of inaccurately formed drill bits results in a significant expense to drill bit manufacturers and therefore a reduction in that rejection rate is desirable. Also, an improvement in the ease of bonding drill tips to drill bits is additionally desirable.
It is an object of the present invention to overcome, or at least alleviate one or more of the above disadvantages. It is a particular object of the invention, to provide an arrangement, in which the rejection rate of drill bits to which cemented carbide tips are bonded, is lowered.